Joined: Jan 2008
Posts: 23,642
Likes: 1,106
From: Mans of NC & SW UT Desert
Bikes: 86 Katakura Silk, 87 Prologue X2, 88 Cimarron LE, 1975 Sekai 4000 Professional, 73 Paramount, plus more
Modern chain is fine. Unless you REALLY have to have an old NOS freewheel, IMHO, I would buy a NEW 5 speed freewheel instead. NOS term tends to be grossly overused, and given to non-NOS parts.
I have on occasion had skipping due to other reasons. Most recently, it was a bent derailleur hanger. It was not obvious! But once I put my tool on it (Park DAG), it was not straight. In this situation, I had a brand new freewheel and new chain, so I knew those parts were good.